A variety of heat shield concepts designed for minimizing heat transfer to a spacecraft structure during atmospheric reentry are known. Due to their high thermal stability, carbon-carbon composites are often used for this purpose. As used herein, the term “carbon-carbon composite” will refer to a composite material containing a discontinuous phase of carbon fibers in a pyrolytic or graphitized carbon matrix. To further increase the thermal protection attributes of carbon-carbon composites, a sacrificial ablative layer can also be present in a desired region of the composite by being layered thereon to form a laminated carbon-carbon composite. Such laminated carbon-carbon composites are often produced by fully densifying the underlying bulk carbon-carbon composite and then forming the sacrificial ablative layer in a separate curing cycle.
To densify a carbon-carbon composite, multiple densification cycles (typically 4-6 cycles) involving resin impregnation and subsequent carbonization are often performed. Each densification cycle reduces the total porosity of the composite, and after full densification, the total porosity of the composite may be about 8-10%. While laminated and densified carbon-carbon composites can often provide excellent thermal protection properties, the aerothermal environment of atmospheric reentry can often create extreme thermal gradients, especially through-thickness, within a laminated carbon-carbon composite. The high thermal gradients can induce various types of mechanical stress, such as interlaminar tension and interlaminar shear, which can lead to delamination and failure of a heat shield. In this regard, conventional laminated carbon-carbon composites can often exhibit relatively poor interlaminar tension (less than 1000 psi) and interlaminar shear properties (˜1500 psi). Untimely delamination can lead to damage and potential loss of a reentry vehicle. Moreover, delamination can significantly impact the aerodynamic smoothness of a reentry vehicle's surface, which can sometimes undesirably impact the vehicle's reentry trajectory and maneuverability.
In view of the foregoing, methods for producing laminated carbon-carbon composites containing an ablative layer and that are more resistant to thermally induced stress in the underlying composite layers would be of significant interest in the art. The present disclosure satisfies these needs and provides related advantages as well.